1 Field of the Invention
The present invention relates to a process for effectively controlling operation of a continuous mixer having an adjustable exit opening which is connected to a melt pump.
2. Description of the Prior Art
Numerous polymer resins, e.g., polyethylene and polypropylene, are produced as very fine particles and are subsequently converted to pellets to facilitate shipping and handling. At this stage of the processing, the resin can additionally be compounded with additives, such as stabilizers, antioxidants, fillers, colorants, and the like, if desired. In general terms, this in-line processing is accomplished by mixing and melting the resin after it has been purged of any unreacted monomer(s) or other volatile materials and forcing the melt through a suitable die plate into a pelletizer. The pelletizer consists of a means for chopping and cooling the extrudate.
More specifically, the resin is continuously fed into a combination mixer/melter having interspaced entrance and exit openings and a means for moving the material therethrough. Combination mixer/melters, sometimes simply referred to as mixers, are known and widely utilized for the continuous processing and compounding of thermoplastic materials. Mixer/melters have a restraining means for controlling the flow of exiting resin. This restraining means which in addition to controlling throughput, creates a back pressure within the mixer which influences the mixing/melting efficiency. Since the temperature within the mixer is directly proportional to the pressure, i.e. temperature increases as more work is performed on the resin, varying the flow rate also serves to control the temperature. Restraint to the flow of material discharged from continuous mixers is generally accomplished through the use of an adjustable exit opening and/or by using a moving surface discharge device which contacts the material with one or more surfaces moving at controlled speeds in the direction of flow. Gear pumps and extruder screws are most widely used to provide such moving surfaces.
Mixer melters are typically low energy systems and are not capable of developing the pressures necessary to achieve acceptable extrusion rates. They are therefore usually coupled with a melt pump, such as a gear pump or extruder screw, which imparts the necessary high energy to the resin melt for extrusion and pelletization. The melt pump also serves to control the pressure and throughput in the mixer as described above.
While the mixer and the melt pump are independently driven, each having their own motor drive, operation of the mixer and melt pump must be carefully matched to avoid problems in the in-line processing system. For example, if too large an amount of resin melt is fed from the mixer to the melt pump, the pump capacity will be exceeded which can create excessive back pressure in the mixer causing the rupture disk to blow. If an insufficient amount of resin is fed to the melt pump, i.e. the gear pump is operated in a starved condition, excessive speeds (rpm) can be generated in the gear pump causing the gear pins to shear. In either instance, the line will have to be shutdown until the necessary repairs can be made. Such disruptions are time-consuming and costly and, if the line is down for an extended period, necessitate shutting down the polymerizer. To minimize these problems, the mixer and melt pump are suitably sized and coupled so that feed from the mixer/melter matches the requirements of the melt pump but does not exceed that necessary to achieve effective mixing and melting of the resin being processed.
Several methods have been employed by the prior art processes in an effort to overcome the problems associated with the continuous in-line processing of plastic materials using a mixer interconnected to a gear pump. For example, U.S. Pat. No. 4,310,251 to Scharer et al discloses a continuous internal mixer which discharges through a fixed-size opening directly and positively to either a screw type extruder or a gear pump. The speed of the gear pump or extruder automatically responds to the temperature of the material discharged from the mixer which is a function of the mixer internal pressure.
U.S. Pat. No. 4,452,750 to Handwerk et al discloses an in-line mixer-gear pump arrangement for processing synthetic thermoplastic materials which employs the pressure developed between the fixed-size exit port of the mixer and gear pump to effectively control the speed of the gear pump. This in turn affects, in proportional relationship, the amount of energy transmitted to the melter/mixer and the temperature of the melt.
U.S. Pat. No. 4,707,139 to Valenzky et al relates to a control system for a continuous mixer having a moving surface discharge device. The internal pressure in the mixer and the discharge of material from the mixer is controlled by the gear pump and the speed (rpm) of the gear pump is controlled in relation to the torque produced by the mixer motor. A change in the torque of the mixer motor results in a change in the rpm of the gear pump, thereby maintaining the necessary relationship throughout the in-line processing operation.
While the above methods generally effectively eliminate the major problems associated with such in-line processing operations, they are not sufficiently responsive to prevent all sudden pressure surges which result from unevenness in the mixing action. While these latter pressure surges are relatively minor compared to the larger catastrophic pressure differentials previously mentioned and do not result in equipment failure and shutdown of the processing line, they do nevertheless adversely affect the quality of the resulting extruded pellets. Primarily, the pellets are not of uniform size and may be irregularly shaped. In some instances, the pellets may even agglomerate. While the oversized pellets and agglomerates will be retained upon screening, they must either be reprocessed or scraped. Smaller-sized pellets will pass through the screens; and, if they are present in sufficient quantity, necessitate downgrading of the resin.
It is therefore an object of this invention to provide a system and method for controlling a continuous mixer having an adjustable exit opening and connected to a moving surface discharge device to effectively control the speed of the discharge device and thereby maintain the necessary relationships throughout the in-line processing operation.
It is a further object of this invention to provide a system and method for controlling a continuous mixer having an adjustable opening and connected to a melt pump which is more responsive to changes and imbalances which develop within the in-line processing equipment.
Yet another objective of the present invention is to provide a system and method for controlling a continuous mixer having an adjustable opening controlled by hydraulic pressure and connected to a gear pump to minimize pressure surges which adversely affect the quality of the extruded pellets and to obtain pellets which are as uniformly sized as possible.